C.N. Marasas, M. Smale, R.P. Singh, and P.Pingali

Introduction

Leaf rust caused by Puccinia triticina (Figure 1) is an important disease of wheat (Triticum aestivum L.) worldwide. The cultivation of resistant varieties is the most economical and environmentally friendly control method. Rust pathogens are able to mutate rapidly and form new races. Genes conferring race-specific resistance produce resistant reactions, but their effects are overcome within a relatively short time. In contrast, genes conferring race-nonspecific resistance have partial and additive effects, which appear to endure longer.

Control of rust diseases of wheat through genetic resistance has been an important breeding objective at the International Maize and Wheat Improvement Center (CIMMYT) (Rajaram et al. 1988). Utilization of the nonspecific type of resistance to leaf rust, controlled by genes that confer slow rusting, has been the dominant breeding strategy used during the past 25 years. This study aims to estimate the global economic benefits of CIMMYT's decision to incorporate nonspecific, rather than specific resistance to leaf rust into spring bread wheat. The analysis is still in progress and the information presented here is therefore preliminary.

Methodology

Breeding for genetic resistance to rust diseases in wheat is an example of research aimed at maintaining crop productivity. Research benefits are valued in terms of the yield losses that would have occurred globally if a strategy for specific resistance, rather than nonspecific resistance had been employed.

• A list of all major spring bread wheat varieties grown in the developing world was drawn from CIMMYT's latest Global Wheat Impacts Survey data, conducted in 1997 by the Economics and Wheat Programs (Heisey et al., forthcoming). Varieties released after 1970, when CIMMYT's nonspecific resistance breeding program was initiated, and for which seed could be obtained, were grown in a field trial in El Batán, Mexico. The varieties were classified using the modified Cobb scale (Peterson et al. 1948) for the type and the level of genetic resistance to the current Mexican population of leaf rust. The trial data, combined with information on leaf rust resistance mechanisms from various trials, were used to classify the varieties by slow rusting category (SRC). Each category was assigned different levels of potential yield savings by CIMMYT breeding mega-environment (ME). These results, combined with the variety area estimates in the Impacts Survey data, provide a sample estimate of the area currently planted by SRC in the developing world.
• Historical logistic diffusion curves for each SRC and ME were fitted using 1) function parameters including ceilings, lags, initial and final years, which were estimated from historical CIMMYT Wheat Impacts data (Heisey et al., forthcoming; Byerlee and Moya 1993); 2) a time series of areas estimated by combining national data on wheat areas obtained from the Food and Agriculture Organization (FAO) with CIMMYT Impacts data on spring bread wheat areas by ME and country; 3) Wheat Program estimates of areas potentially affected by leaf rust in each ME; and 4) the sample estimate of 1997 percentage areas by SRC and ME.
• The yield savings per SRC and ME were estimated for four different scenarios of yield losses using 1) the yield saving of each SRC over the losses suffered by susceptible varieties, and 2) a time series of average yields estimated by combining FAO national wheat yield information with CIMMYT Impacts data on spring bread wheat yields by ME and country.
• Production savings are being calculated by combining the yield and area time series generated by the above two steps. The net present value and internal rate of return associated with these savings will then be computed using the real wheat export parity price. Scenarios will be simulated to represent different assumptions about actual yield savings, alternative investments, and the costs of the program in order to test the sensitivity of the results.

Preliminary Results and Discussion

Table 1 shows the percentage area of the sample varieties per SRC and ME.

• The major proportion of the sample area was protected by genes conferring nonspecific resistance. Thirty seven percent of the area was planted with varieties showing moderate resistance (SRC 3) and a further 37% of the area was planted with varieties showing high levels of resistance (SRC 4 and 5). These varieties should survive most leaf rust epidemics.
• Ten percent of the sample area was protected by genes conferring specific resistance (SRC 6). The percentage area planted with varieties in SRC 6 was the highest in MEs 4b and 3. Characteristics other than nonspecific leaf rust resistance might be more important in these MEs. However, these varieties comprise only a relatively small proportion of the total sample area.
• Only 10-16% of the sample area was planted with varieties showing moderate to higher levels of susceptibility to the Mexican population of leaf rust (SRC 2 and 1, respectively).

Research conducted at CIMMYT thus far indicates that the economic benefits of breeding for nonspecific resistance to leaf rust in spring bread wheat should be substantial (Sayre et al. 1998; Smale et al. 1998). For the Yaqui Valley of Mexico alone, the internal rate of return on the research investment over the period 1970-90 was estimated at 13% under the most conservative assumptions. The benefits expressed in 1994 real terms amounted to US$ 17 million. In enlarging the scale of analysis from the Yaqui Valley to CIMMYT's global mandate area, the benefits are expected to increase substantially.

References

Byerlee, D., and P. Moya. 1993. Impacts of International Wheat Breeding Research in the Developing World, 1966-1990. Mexico, D.F.: CIMMYT.

Heisey, P.W., M.A. Lantican, and H.J. Dubin. Forthcoming. Assessing the Benefits of International Wheat Breeding Research in the Developing World: The Global Wheat Impacts Study, 1966-1997. Mexico, D.F.: CIMMYT.

Peterson, R.F., A.B. Campbell, and A.E. Hannah. 1948. A diagrammatic scale for estimating rust intensity of leaves and stem of cereals. Can. J. Res. Sect. C 26:496-500.

Rajaram, S., R.P. Singh, and E. Torres. 1988. Current CIMMYT approaches to breeding for rust resistance. In N.W. Simmonds and S Rajaram (eds.), Breeding Strategies for Resistance to the Rusts of Wheat. Mexico, D.F.: CIMMYT. Pp 101-118.

Sayre, K.D., R.P. Singh, J. Huerta-Espino, and S. Rajaram. 1998. Genetic progress in reducing losses to leaf rust in CIMMYT-derived Mexican spring wheat cultivars. Crop Science 38: 654-659.

Smale, M., R.P. Singh, K. Sayre, P. Pingali, S. Rajaram, and H.J. Dubin.1998. Estimating the economic impact of breeding nonspecific resistance to leaf rust in modern bread wheats. Plant Disease 82: 1055 - 1061.

© CIMMYT April 2001

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